I'm on a non-premium account, and I'm not even using HTML.
I woke up this morning and my homepage footer was gone, and there's no longer a way to navigate to my previous posts beyond what's on the front page.
http://justreboot.wordpress.com Twenty Eleven theme
I have no idea how to resolve any of the issues shown in the markup validation service - they all seem to involve data I have no access to.
I recently deleted a bunch of tags and categories I deemed unnecessary, and posted an embedded YouTube within a post - other than that, can't guess what is causing this. I even removed the YouTube posting to see if it would help, and it didn't.